There exist several well known transmit diversity techniques wherein one or several redundancy versions relating to identical data are transmitted on several (at least two) diversity branches “by default” without explicitly requesting (by a feedback channel) further diversity branches (as done in an ARQ scheme by requesting retransmissions). For example the following schemes are considered as transmit diversity:                Site Diversity: The transmitted signal originates from different sites, e.g. different base stations in a cellular environment.        Antenna Diversity: The transmitted signal originates from different antennas, e.g. different antennas of a multi-antenna base station.        Polarization Diversity: The transmitted signal is mapped onto different polarizations.        Frequency Diversity: The transmitted signal is mapped e.g. on different carrier frequencies or on different frequency hopping sequences.        Time Diversity: The transmitted signal is e.g. mapped on different interleaving sequences.        Multicode Diversity: The transmitted signal is mapped on different codes in e.g. a CDMA (Code Division Multiple Access) system.        
There are known several diversity combining techniques. The following three techniques are the most common ones:                Selection Combining: Selecting the diversity branch with the highest SNR for decoding, ignoring the remaining ones.        Equal Gain Combining: Combining received diversity branches with ignoring the differences in received SNR.        Maximal Ratio Combining: Combining received diversity branches taking the received SNR of each diversity branch into account. The combining can be performed at bit-level (e.g. LLR) or at modulation symbol level.        
Furthermore, a common technique for error detection/correction is based on Automatic Repeat reQuest (ARQ) schemes together with Forward Error Correction (FEC), called hybrid ARQ (HARQ). If an error is detected within a packet by the Cyclic Redundancy Check (CRC), the receiver requests the transmitter to send additional information (retransmission) to improve the probability to correctly decode the erroneous packet.
In WO-02/067491 A1 a method for hybrid ARQ transmissions has been disclosed which averages the bit reliabilities over successively requested retransmissions by means of signal constellation rearrangement.
As shown therein, when employing higher order modulation formats (e.g. M-PSK, M-QAM with log2(M)>2), where more than 2 bits are mapped onto one modulation symbol, the bits mapped onto a modulation symbol have different reliabilities depending on their content and depending on the chosen mapping. This leads for most FEC (e.g. Turbo Codes) schemes to a degraded decoder performance compared to an input of more equally distributed bit reliabilities.
In conventional communication systems the modulation dependent variations in bit reliabilities are not taken into account and, hence, usually the variations remain after combining the diversity branches at the receiver.
The object of the invention is to provide a method, transmitter and receiver which show an improved performance with regard to transmission errors. This object is solved by a method, transmitter and receiver as set forth in the independent claims.
The invention is based on the idea to improve the decoding performance at the receiver by applying different signal constellation mappings to the available distinguishable transmit diversity branches. The idea is applicable to modulation formats, where more than 2 bits are mapped onto one modulation symbol, since this implies a variation in reliabilities for the bits mapped onto the signal constellation (e.g. for regular BPSK and QPSK modulation all bits mapped onto a modulation symbol have the same reliability). The variations depend on the employed mapping and on the actually transmitted content of the bits.
Depending on the employed modulation format and the actual number of bits mapped onto a single modulation symbol, for a given arbitrary number (N>1) of available diversity branches the quality of the averaging process is different. Averaging in the sense of the present invention is understood as a process of reducing the differences in mean combined bit reliabilities among the different bits of a data symbol. Although it might be that only after using several diversity branches or paths a perfect averaging with no remaining differences is achieved, averaging means in the context of the document any process steps in the direction of reducing the mean combined bit reliability differences. Assuming on average an equal SNR for all available diversity branches, for 16-QAM 4 mappings (4 diversity branches) would be needed to perfectly average out the reliabilities for all bits mapped on any symbol. However, if e.g. only 2 branches are available a perfect averaging is not possible. Hence, the averaging should then be performed on a best effort basis as shown in the example below.